Acoustic range finder



Dec. 12, 1950 w. A. MUNSON 2,533,499

ACOUSTIC RANGE FINDER Filed Aug. 30, 1944 2 Sheets-Sheet l /70 nvrmcomvecrma TELE. LIA/ CHANNEL 8 m w ATTORNEY Patented Dec. 12, 1950 ACOUSTIC RANGE FINDER Wilden A. Munson, Chatham, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 30, 1944, Serial No. 551,831

3 Claims.

This invention relates to acoustic range finders of the general type disclosed in'my copending joint application with J. C. Steinberg, Serial No. 509,710, filed November 19, 1943, now Patent No. 2,418,136, issued April 1, 1947, in which a sound source, such as an enemy gun, is located by means of bearing lines each of which is determined from differences in the arrival times of the sound at a plurality of pick-up points spaced about a pick-up position.

The object of this invention is an improved range finder of this type which is adapted to be made up in relatively small units which are readily portable and convenient to use.

According to this invention the pick-up from all the microphones at each pick-up point is recorded on a single loop type of magnetic recorder and this recorder together with the necessary reproducing and arrival time difierence measuring apparatus forms one compact, mechanically independent unit of the range finder. Each such unit is located close to the corresponding pick-up point, the necessary electrical coordination between the several units being obtained by interconnecting telephone lines.

As in the range finder of the copending application referred to above, the outputs of the microphones are continuously recorded and erased until the sound of interest is heard at which time the recorder is stopped to preserve the several recordings of the sound. In the present invention, however, the several recordings are in diflerent portions of the same tape or wire and they are reproduced simultaneously in pairs to actuate an oscilloscope. These simultaneous reproductions form Lissajous figures from which the arrival time differences may be determined by measuring the displacement of one of the reproducing magnets required to cause the Lissajous figure to assume a given form, such as a straight line.

In the prior range finder the record members were power driven during reproduction and the reproduction was confined to the portions of the recordings containing the sound of interest by the use of commutators. According to an important feature of this invention the extraneous noise is eliminated during reproduction without the use of commutators thereby both avoiding the difilculties commonly encountered in using commutator contacts and at the same time simplifying the apparatus and reducing its weight.

Suitable means, such as a handwheel, is provided for moving the tape untilthe signal is seen on the oscilloscope. The wheel is then oscillated about this point to give successive reproductions while the arrival time difierence is being measured and this operation is further facilitated by blanking out the return traces by means of a circuit disabling switch operated by the reverse motion of the handwheel.

The accuracy with which the sound source is located is, of course, impaired by any phase distortion produced in the recording and reproducing amplifiers and the efiect of any such distortion is further increased if the same amplifier is used for both recording and reproducing the output of any given microphone. According to a further feature of this invention the effect on the final measurements of any distortion in the amplifiers is eliminated automatically by means of amplifier switching circuits operated in the course of the recording and reproducing operations. These circuits are such that in reproducing any selected pair of recordings use is made of the same pair of amplifiers as were used in making these recordings but the amplifiers are interchanged so that each recording is made with one amplifier of the pair and reproduced with .1 the other. In this way the phase distortion produced by both amplifiers is present in both reproductions and is therefore balanced out in measuring the time difierence between them.

These and other features of the invention including novel methods of ranging will be clearly understood from the following detailed description and the accompanying drawing in which:

Fig. 1 shows the recording and reproducing circuits of one unit of the range finder;

Fig. 2 is a diagrammatic view of the apparatus necessary for operating the invention;

Fig. 3 is a speed factor chart which is useful in operating the apparatus; and

Fig. 4 is a diagram for explaining the construction of the chart of Fig. 3.

In the drawing, the microphones l and 2 are accurately spaced equidistantly from the microphone 3 along lines forming a right angle facing in the general direction of the sound source to be located as shown in Fig. 2. During the recording operation the microphones are connected to the recording magnets 4, 5 and 6 by means of recording channels indicated generally in Fig. 2 p

as A, B and C and in more detail in Fig. 1 to be described.

The recording magnets cooperate with a loop record member, such as a tape I mounted on drums 8 and 9 and driven by a motor generator unit [0 through a suitable clutch and speed reduction mechanism ll. Since, the apparatus is amplifiers 26A, 28B and 29C, gain control stages ZIA, 2IB and H and conventional power; amplifiers 22A, 22B and 22C. The. outputs; of ther ain control stages are also used to drive bothithe' power amplifiers 23A and 23B.for,the oscilloscope 24 and the amplifiers 25A andfliiBfor the'headset 26.

A Hartley type vacuum tube oscillator 27, in

whichfih diw re l i l wh h. s e t dbei .tweee w ml id. e ia 9 tgmis -po n nc nn d t the h e. .1 tuned by .a condenser l 0 6. in. theiusual. manner,

output-.of high frequency such as 10,000 cycles per; second. which is used either to energize theerasing magnets ,l 1;, 18 and .i Q and bias. the, re-

ordi ma -n t 4; fi fi-Qf a ys e e q y ezshe ninl ate 2323 1132 e O d d Ql-W.

ers n a a-l erna ne in ut t .ait n m r 2 8 9Q 1 na hemsi ion. o sw hs i G T ig Ye -a ut ut at eifir nsf t is r fi l-1 fied: by the. tube islandappl-ie d over. conductors. -.a.n. 3 to-a he v a e d vidi n r 32 which in turn prcvides the. veryv high voltages necessary, for 1the;- defiectingPplates Bl; 168 and the; aocelleratingl electrodes I89, l 19 of the oscilloscope: 24-... The. transformer- 23 is also-provided with a: low; voltageqoutput; coil: for heating; thev filament of the rectifier tube in the conventionalmanneras .-shown.

In.Fig -.1 all;si gnal circuitswitches are shownin; the-= extreme. left or recording position; Many ofthe? switches; although showninv different. lo cations in:the :drawing are'in practicegangedion convenience; operation; For, example; the switches-8 A; SIB and Sic are gangedtogether with all; the? other S1 switches; namely, SID to. S1J ito :form? a; master control .31 for. conditioning;

the circuits for; recording-.01" reproducing. Vol-=- umewcontrols S;- (A;- B; and C); are: gangedtogethe'r to change the volume levelsdn all'thre'e. channels; simultaneously when; recording" and; a similar functionis: performedzwhen reproducingbjy'the gangedreproducing: controls S4 (rig Bland C). These volume-controls are selectively con-- nccted in the cathode return; leadsi;=of the gain control tubes 2| (A, BandqC); under controltof the. switch 51 as shownlso. that thechannels may bev-bal ance'drand adjusted to. di fierentgains for recording:and-reproducingandtheseadjustments left; undisturbed when transferring. froml -one. operation; to thepther. The. switches1;S zA;v and $2B -are also. ganged. and serve to connect the headphone amplifiers Y 25Av and Z EB selectively tov any: one; off the threechannels for. monitoring purposes during: recording. or; to a telephone line. to receive messages from a telephone sub-set-associated withanother unit of: the. range finder.

Recording 1 wa l the motor-gen rator rs, operating and all. other switches in the recording position as shown, the'picli-up of microphones i, 2' and 3'is continuously recorded by the magnets 4, 5 and 5 respectively on the tape 7 each recording being preserved only until it reaches the next erasing magnet ll, H3 or I9 so that at any given instant the tape will contain a record of the sounds received in the preceding short time interval. The microphones 4;; and 3 are connected to. their respectivei-imagnets l, 5 and fi through identical networks indicated in Fig. 1 as channels A, B and C, the elements of which are numbered alike except for the channel designation A, B or C. A descriptionof the recording operation of channel A,, which follows,-.will likewise be applicable to ,cl an l'iel.:13Tori);v

During thesrecording operation of channel A,

the sound to be recorded is picked up by micro- 'phone F and" pre amplified by amplifier 26A iliA'i condenser WA is shunted across" the screen" grid? resistorj18A- to" prevent feedback of, the alternating currentcomponent of i'tiie screen grid-current? 7 V The amplified sound signal. fromamplifi'er is transmitted togain controlitub'efll yrrieans of couplingcondenser l lA- tliroughanunput"net? work comprising' control grid *resist'or leA arid a? standard filter arrangeme'ntj resistor" 56A and condenser-17h. n variable resistor is ntrolled switch 83A-and is connected in-'s 'eries*w itli* the" th'ode" of tube 2-| through sw mmer; e rev e the necessary" self 'bias' voltage fdit the control grid? of the' tuber- Thevolurne genie sound signalfrorii' amplifier ZBA ma y bef adjusted by switch SBA byselecting l different -re'sistoi stagesfin theswitch: adjustedsouii'd signal is transmitted 'fromgain controltubfi lA to mm pus-er 2 2iA*-by-- means- 0 couplingcondenser 802% through an input network comprising control grid resistor 81 A and afilter consis-ting' of resistor and coridens'er 83E. I The anoplified soun'd signaHroi'ir -amplifier 22W is transmittedb means of coupling condenser S t o' transform r 35A which'is inductively cou= pied to=-recorder 4': The sound signal is tl'ie'n re: c'o'rdedbyre'corder= 4 on"tape- 1.

In the diaw ing; asshowri in -Fig: 1", Switch S22 9 connects the-monitor-ing and oscilloscope ani'pli fiers 2 33A: 2 33 anew-5A;- 2513 te -cl-iannel A for" monitoring purposes during reoording swam S2A SiV'S- to conlle dt the headset and DSC11163 scope amplifiers-.to= any one of the threecha nels-during recording operationst nioni t'oring. and oscilloscope;amplifiers; and:-

are connected in parallel to swit'c'h S1 E1- In the same way; amplifiers 23B andi 25B are connected to svv'ii' ch-SIFI With switch SF i'rr it's fir'st o1 reeordii gi position'; the-amp'ylifiersfit'l ii 25A; 23B and 25B- are connected to the movablamonitor ing switcli stir-which is coup'ld to gain control tube z l'zrby condenser arm The} osciuoseope and headset amplifiers ZSA- andIZ'SE l are com nect'ed' in parall'ehtoswitch Sl'E" through} a con 31mg network comprisingj condenser Bin; resist we, sea which serves to eliminate high: frequency sound waves from. the oscilloscope 24" and the headset. 2's: Thein'put netw rk'tc the oscilloscope. amplifier 2i3A consists of a filter arrangef8 Ine'nt, condenser A, resistor 94A and control grid resistor 93A, connected to switch SIE through the filter 31A, 88A. A similar filter and input network, as described above, connect the oscilloscope and headset amplifiers 23B and 253 to switch SIF. Plate voltage is supplied to the monitoring and oscilloscope amplifiers 23A, 23B and 25A, 2513 through plate resistors 96A, 95B and 9| A, MB, respectively. Headset 25 is inductively coupled to transformers 50A and. 9813 which are connected to amplifiers 25A and 25B by..coupling condensers 83A and 89B, respectively.

Oscilloscope 24 is coupled to the amplifiers 23A,.

and 23B by coupling condensers 92A and $23 which are connected to the plates Hill, I93 of the oscilloscope. The sound signal during recording operation, therefore, may be observed on the oscilloscope 24 or monitored by the headset 2B.

.When the sound of interest is heard, recording is stopped immediately to preserve the record of this sound as made by all three channels. This may be effected by closing a switch 33 to energize within the housing II, a clutch magnet which disconnects the drum 8 from the drive and stops the motion of the tape. In practice it is found suficient to provide a storage time of only two seconds for each channel and this is conveniently obtained with a total tape length of only four feet and a tape speed of six to seven inches per second. In practice the switch 33 could be used also to energize over an interconnecting telephone line the clutch magnets of all other recording units forming part of the range The main control switch S1 is then rotated clockwise from the recording position in which itis. shown to the proper position to select the desired pair of recordings for reproduction. For example, if the recordings made by magnets 3 and 5 from the pick-up of microphones l and 2 are to be reproduced, the switch is moved to the second position. In this position the reproducing magnet I4 is connected through switches 81A and $13 to channel B and the reproducing magnet is connected through the same switches to channel A thereby interchanging the amplifiers to balance out the amplifier distortion as explained above.

Since the tape moves at'low speed it comes to rest almost immediately when the clutch is disengaged and the operator very soon learns about how far back the tape must be moved to reproduce the desired signal. The handwheel 34 is rotated clockwise by the estimated amount and then moved back and forth through a small angle while watching the screen of the oscilloscope tube 24. On the backward motion of the tape the friction of the pad 35 on the handwheel rotates the arm 36 about the pivot 31 thereby closing the contacts 38, 39 and 49 and connecting conductors ll, 42 and 43 to conductor M.

This short-circuits the reproducing magnets l4,

l5 and i6 and prevents any reproduction during reverse tape motion. If the signal is not found the tape is moved further back and the back and forth motion of the handwheel is repeated until the desired signal appears.

In the case of any ambiguity as to the identity of a particular signal which is reproduced, the

main control switch S1 may be set to the forth or monitoring position in which case each reproducing magnet is connected to its associated amplifier channel at the monitoring contacts of switches $15,813, and S10 and the monitoring and oscilloscope amplifiers 23A and B and 25A" and B are connected in. parallel to the arm of the monitoring switch SzA by the switches SIE and 51F. With the monitoring switch on channel A as shown all three recorded signals may be reproduced over channel A and observed in succession by means of the headset 2% or the oscilloscope or both if desired.

' In Fig. 2 it will be noted that the spacing between the erasing magnet l8 and the recording magnet 5 is much greater than the corresponding spacing of the magnets I1 and IS with respect to the recording magnets t and 6. In the recording operation all signals and noise are erased from the portions of the tape which at anygiven time are located between an erasing magnet and the next recording magnet. Due to the location of the magnet 18 the silent interval after the reproduction of the channel B signal will be much longer than the other silent intervals and hence it may be used as a reference for. identifying any:

of the signals.

Since, as stated above, all the S1 switches are;

ganged the opertion of the switch to reproduce any pair of recordings such for example as those; made by microphones I and 2, will also perform. all the other switching operations required for,

reproduction. At switches SiH, S11 and SJ the reproducing volume controls S4 (A, B and C) are substituted for the recording controls S3. (A, B and C) to adjust the gain to the proper value for reproduction. The plate supply for the arnplifiers 22 (A, B and C), which are not required for reproduction, is cut off by switch SiDi, the A monitoring and oscilloscopeamplifiers are con.-.

nected to channel A and the corresponding B amplifiers to channel B by switches S113 and S11 over conductors 49 and 50 and plate power is supplied to the oscilloscope by switch SIG.

As the handwheel 34 is moved back and forth to give successive reproductions of the selected pair of recordings, the oscilloscope usually will.

may be indicated by adjusting the pole piece until the two receivers of the headset 26 are in binaural balance or if desired this aural technique may be used as a check on the operation of the oscilloscope. When this condition has been obtained the readings of the associated counters 4'! and 48 provide a measure of the difference in the.

arrival times of the signal at the microphones I and 2.

This procedure may then be repeated to measure the difference in the arrival times at microphones I and 3 by moving the control switch S1 to the third position, operating the handwheel as before and in this case adjusting the micrometer screw 35 as required to give siniuitaneous reproductions. switch 81A and S10 connect the reproducing pole-pieces l4 and I6 to channels C and A, respectively, to balance out distortion and switch Sil switches the amplifiers 23B and 25B from th B to the C channel over conductor 5|.

. The control switch S1 may then be moved to its fifth position to connect the magnets l5 and I6 to the oscilloscope over channels 0 and B, respectively in which case the amplifiers 28A and Under this circuit condition.

se aw iiu tsare' disconnected" fromrtl'iez. fimhannelrzandx'z' reconnected 'to thexCt. channek ove conductor tl'ierebyainterc-hanging; he "channelziecnneetionsr medium-making-thisvpa -oiitrreccrds;v The ..mi:r crom'eten' screws dfii isathema readjustedate give simultaneousreproductions:: of": thee recordings-'1 meda -by microphones :1 and Kit-and: counter; readings are noted as before;

The' operator then" v has: three:.zsetss of counter readihgsvrepresenting: theerelative-narrivali timesem :in-ewhich: theasca'lesuare; calibratedsin terms:

ofiith sound;fatt-each: of tl'ie threeemi'orophoness and. rum: th'sse; readingshen'iszaab'lavin know-n11 manner: to :determineathe direction: of". thew-sound sourc'eiromrthi's particular:pickempipositicm :Oner ortm'or 'similar aunitsiidisposedraat: oiheliip points-5 give-1: similar?" directi'cnzz; lines-1. from'szthosea; pcints'wto the sourceand; knowing rtiiesrelativeiz locations; :of- 1 the: pick-upn points;v the source readilylo'cated' by well-known;graphioalimethodss.

a 35.. one the.,ty.pical circlersegments shown arfi -nthe ole-up: 151; microphones :s land: I zthe wave will arrivefirs 8 air and: usingrthesercounter zreadingssin con junctiom-rwith thezchartwoi: Ffign3z; In':this;:char thez scaleszare calibratectimterms:oflthemcoimtfi readings-sandalthe.=numbers; such as.;;45,; 40418.11 L

The:..-cnstruction: and userof; theicliartr will be more easily understood byefirst considering Fig;

ccunter -readings the -m-icroph0ne:. array represented:loy the pcints:. I 2; and :3: Foria at: microphone I: and" then some; time-latenlt; will arrive-simultaneously atumicrophones; 2- and 3:. Witha given' veloc-ityof'soundpropagation and tape speed the difference-inthearrival times-at:

Infcases wherer-ittis inconvenient: loniimpraotiez 7 gm; microphones" l and 3 will give some definite reads ings'such as 100 on counter-41 and since there'tis noydifference-dn arrival =time-at microphones- 2;- anrl 3 counter -43 wi'll -reacl zero? readingsare the coordinates ofpoint--5 6 :on the line fil -to the sound-sourcee Similarly; awave-- approaching the-array from a sourceon the linefiilgives areading of zero on c0unter-4 l and=a reading of" 10% on -counter 8 thereby definingpoint-59.

: A 'WaVe approaching from' some intermediate direction such -ior--example as along the line will give readings ofand" 6'O'-respectively; thereby defining point 6! on the chartand waves-- from any other directionwill'jgive other similar :thentfiredfirommh ':positioniofitheothertBfiipointsall ofwhich willlie on the circle 62 vas longjas the velocity pf sound and tape. speediiare constantz' For any other'set of values offisounct velocity and tape speed all of these points will" lie on other circles of greater, or lesser; diameters nectihgi-lineztoterminal 52 of s'witchrSzB and'over mid'epending on whether the, variations in ;sound"' oonduoto :53 to; the ihputz'of-izthe 'Czlchannel to: be; recordedwmr therstapeabythea magnet fi; i sishnrt: time; laterz'th'eQ-SQuni' of: the: shot; transmitted: through the air: is :pi'cke'dtzup r'byr microphones i velocityand tape speed'jresult' in greater or lesser, spacings in the tape between the ,successiverree cordingsoiithe soundli.

Itiwill'. therefore .be seen thatpan-y particular;

S aIIdLB? Withi'thmcontrol switeha-Snset tmmonitor .(pos siti0m:;49;1: and; the 5; monitor? .svvitclzLfiZAaseti. to

monitoneonzzchannel Cf. the atapexis, scannedrtn i-wfind crating condition. and; may. therefore bedesig;

natedbyanumber representing the properefactor, by which the number representing the difference in. handwheel lreadingsl. ,ref'erredhtol above must thQiI word; ofaxthesshot aslrecordedscventhe:teleimi bemultiplied inordertorcbtainanraccuratemeasr.

phon ne and :ther-positicn :of the:- handiwheelz; likirimreproducingethis :record is znotedtiby; read i ingcthescaleJikiv-itharespectto*thefindeiefit'u The tapexisthen scanned againrto findaandxreproducm theiGaJchanneh'reecrdingeofizthexshotiasspickedfupi or; thereforeasegmentlof the, circle representing their byi microphonei3 andagairrrrecordtis made-rot 171183 heindivheeLpositibna .l Thesereadingsiimayzbep forsexample;;,1.55 :orr theztelephone signahand59fz00a on the microphone signal. For any given velocity" locus .of the point defined.-

successive pairs of v. counterrreadingsobtained, byrmeasurements on; a: sound sourceeapproaching the pickup "point at}. different ang1es+ of incidence. While only three offiasonndi'propagation andrtape: speedatheg difier- 60 1 Segments r h n ast many; other interr.

enact-vine thesetwo handwheel. readings; int. this: 085837145; issdirectlycpnoporticnal to :the distance tmthe' otherunitswliere'zthe shot'was fired HOW=- even sineerneither-ithe velocity of soundi'tonzthei reeordmg; speed can be I assumed. to be :known, thee-.165;

difference in: readings: ;must';,-be multiplied-thy:- a1: factom: correspondingegto the atmospheric: cone- 1: ditions and tapeyspeed'ratc'the time the-smeasureaments: were 2 madezain orders: to determine the;

actual iidistanceawithetheirequirediidegreesof;aca 1 07 51' isvfirst'isel? fi rd position to reproduce curacy.

This-{11f actors" is; obtained by: making;- counter measurements-,2imthermannersqalready :descrihedii Ofiil? ezz-arrivali timer; differences between? tliezzreetherecordingscit-the shot made by: microphones; I and? 3-.andi'the'.arrival time difierence is measurede byadjusting the-screw rw to obtain a read ing onrcounter 31'. The: switchis then: moved-tor cotclings of r the soundi-cef the slrct asitransmittedalt itaiifth: position and a corresponding readin obtained on counter 48 for the arrival time difference between the recordings made by microphones 2 and 3. I

If for example, these counter readings are 060 and 080, respectively, it is found by reference to Fig. 3 that these readings are the coordinates of a point on the speed factor curve 49. The distance between the pick-up point and the other unit at which the shot was fired is therefore This surveying procedure may then be repeated with respect to other units of the range finder until all required distances have been determined after which the system is in condition for ranging on hostile guns or any other sound source in the manneralready described.

What is claimed is:

1. In a sound ranging system, a plurality of microphones in spaced relation with respect to a source of sound to be located, a magnetic recording member, a power drive for the member, a plurality of translating magnets spaced along the member for making and reproducing a plurality of recordings of the same sound as picked up by the several microphones, means for moving the member back and forth with respect to said magnets to cause repeated reproduction of corresponding portions of said recordings, means effective during the backward motion of said member for disabling the reproducing channels, means for displacing one of the magnets along the member to efiect reproduction simultaneously with the reproduction of the corresponding por" tion of another recording, means for comparing the reproductions and means for registering the magnet displacement required to produce the simultaneous reproduction whereby the difference in arrival times and hence the location of the sound may be determined.

2. In an acoustic ranging system for determining the location of enemy guns from the sound of their fire, a base line, and a sound recording and reproducing means at each extremity of said base line, means for establishing the factors of the length of said base line comprising at one extremity of said base line means for producing a test sound at the other extremity of said base line one of said sound recordin and reproducing means which comprises a group of spatially related microphones, a magnetic recordin member, a power drive for said member, a pluralit of translating magnets spaced along said member for making and reproducing a plurality of recordings of the test sound produced by said sound producing means as picked up by said microphones, means for moving said member back and forth with respect to said magnets to cause repeated reproduction of COIIESpOHdlIlg; portions of said recordings, means effective during the backward motion of said member for disabling the reproducing channels, means for displacing one of said magnets along the member to eifect reproduction simultaneously with the reproduction of the corresponding portion of another recording, means for comparing the reproducticns and means for registering the magnet displace ment required to produce the simultaneous it production whereby the difference in arrival times of said test sound as received by each of said microphones and hence one of said factors may be determined, said factor being a value estah lished by a point defined by coordinates representing the difierence in arrival times of said test sound as received by each of said microphones, said point serving to select one of a predetermined series of circles each of which corresponds to a particular combination of sound velocity and recording member velocity, each'of said circles being defined by points the coordinates of each of which represent the difference in arrival times of a sound as received by each of said microphones at a particular angle of incidence, a transmitter at the extremity of said base line where said test sound is produced, a telephone line electrically connecting said transmitter to one of said translatin magnets for transmitting said test sound telephonically to said translating magnet and recording said sound on said recording member, and means for measuring the difierence in arrival times of said test sound as received acoustically and telephonically at said translating magnet and recorded on said recording member, said difference in arrival times being the other of said 'factors establishing said base line, means for determining the bearing of the sound of enemy gun-fire with respect to said established base line, said determining means including said sound recording and reproducing means at each extremity of said base line, each of said sound recording and reproducing means comprising said group of spatially related microphones, said magnetic reccrding member, said power drive for said member, and said plurality of translating magnets spaced along said membar for making and reproducing a plurality of recordings of said sound of enemy gun-fire as picked up by said microphones, and means for displacing one of said magnets along the member to synchronize the reproduction of one recording with a corresponding portion of another recording of said sound, the extent of said displacement being a measure of the difierence in arrival times of said sound of enemy gun-fire as received by each of said microphones, from which may be determined, at each group of microphones, the bearing of said sound of enemy gun-fire with respect to said established base line.

3. Apparatus for establishing the factors of the length of the base lin in an acoustic ranging system including a base line and means at each extremity of said base line for recording and reproducing sound, which comprises, at one extremity of said base line means for producing a test sound, at the other extremity of said base line a plurality of microphones in spaced relation with respect to the source of said test sound, a magnetic recording member, a power drive for said member, a plurality of translating magnets spaced along said member for makin and reproducing a plurality of recordings of the test sound produced by said means as picked up by said microphones, means for movin said member back and forth with respect to said magnets to cause repeated reproduction of corresponding portions of said recordings, means effective during the backward motion of said member for disabling the reproducing channels, means for displacing one of said magnets along the member to effect reproduction simultaneously with the reproduction of the corresponding portion of another recording, means for comparing th reproductions and means for registering the magnet displacement required to produce the simul taneous reproduction whereby the difference in arrival times of said test sound as received by each of said microphones and hence one of said factors may be determined, said factor being a value established by a point defined by coordinates representing the difierence in arrival times of said test sound as received by each of said 

